or to build a web app in Pythonista, while testing it in Safari.browsing

This is what it says on "http://omz-software.com/pythonista/" how can one achive this without loading the code up to a webside. I'd like to open from safari a script in pythonista whereas the output of the skript is shown in safari ...

Using the bottle module, you can run a web server that uses a local IP address. With split view, you can go to that server in Safari. That's all that means. To make a Python script appear in Safari you would have to read the file, then put it in an HTML document and use that in bottle. See the documentation.